Lithium ion secondary batteries are in widespread use as compact, lightweight secondary batteries with high energy density. For lithium ion secondary batteries, for example, lithium metal, lithium alloy, metal oxide, carbon, or the like is used as a negative-electrode material, while lithium-metal composite oxide with a layered structure is used as a positive-electrode material. As an example of such a positive-electrode material, there is disclosed a positive-electrode active material that exhibits high thermal stability when used for a positive electrode of a nonaqueous-electrolyte secondary battery, and has a high charge-discharge capacity (see Patent Literature 1 below).
The positive-electrode active material for a nonaqueous-electrolyte secondary battery described in Patent Literature 1 contains a powder of lithium-metal composite oxide represented by the general formula: LiNixM1−xO2 (x in the formula satisfies (4-Z)×x≥0.75, where Z is the average valence of Ni, and M in the formula represents at least one element whose average valence in the entire M is greater than or equal to 3). When a nonaqueous-electrolyte secondary battery that uses such a powder as a positive-electrode active material is charged up to a composition of Li0.25NixM1−xO2, the number of moles of quadrivalent Ni becomes less than or equal to 60% of the total number of moles of Ni and M.
According to Patent Literature 1, using the aforementioned positive-electrode active material can prevent a decrease in the initial capacity of the battery that would occur due to substitution of Ni with another element, and can, when the material is used for a positive electrode of a lithium ion battery, improve the thermal stability of the battery as long as the material satisfies the following conditions: the number of moles of quadrivalent Ni, which is thermally unstable, becomes less than or equal to 60% of the total number of moles of Ni and the added element M when the battery is charged up to a composition of Li0.25NixM1−xO2 that indicates the fully charged state.
There is also disclosed a method for producing surface-modified lithium-containing composite oxide that contains lithium-containing composite oxide particles and lithium-titanium composite oxide represented by the general formula: LipNxMyOzFa on the surface layers of the particles (see Patent Literature 2 below). In the general formula, N is at least one element selected from the group consisting of Co, Mn, and Ni, and M is at least one element selected from the group consisting of a transition metal element other than Co, Mn, and Ni; Al; Sn; and an alkaline-earth metal element, and satisfies 0.9≤p≤1.3, 0.9≤x≤2.0, 0≤y≤0.1, 1.9≤z≤4.2, and 0≤a≤0.05.
In the production method in accordance with the invention described in Patent Literature 2, a powder of lithium-containing composite oxide is first impregnated with a solution containing a lithium source and a titanium source dissolved therein. Then, heat treatment at 400 to 1000° C. is applied to the obtained lithium titanium-impregnated particles. The invention described in Patent Literature 2 is characterized in that the titanium content in the surface layer of the surface-modified lithium-containing composite oxide, which is obtained through the heat treatment, is 0.01 to 1.95 mol % relative to the lithium-containing composite oxide that is the base material. Accordingly, in Patent Literature 2, a method for producing surface-modified lithium-containing composite oxide is provided that can be advantageously used for a positive electrode of a lithium ion secondary battery, has a high discharge capacity and volume capacity density, is highly safe, and has excellent charge-discharge cycle durability, rate characteristics, and low production cost.